Skin color in tomato fruit is determined by the Y gene. Gene Y produces a distinct, yellow pigment suffused throughout the cell walls of the epidermis of the fruit, whereas its allelomorph, y, produces a transparent or colorless condition in the epidermal walls (E. W. Lindstrom, 1925, Inheritance in Tomatoes, Genetics, issue 10(4) pp 305-317).
Pink tomato fruit is very popular for consumption in Asia. The pink fruit was first described in fruit with a transparent epidermis lacking a yellow pigment (Lindstrom, 1925, Inheritance in Tomatoes, Genetics, issue 10 (4) pp 305-317). Genetic studies revealed that pink fruit result from the monogenic recessive y (yellow) locus present on chromosome 1, while red-colored fruit have the dominant Y allele (Lindstrom 1925). The Y gene has been identified as MYB12 (Ballester et al, vide infra).
The color of tomato fruit is mainly determined by carotenoids an flavonoids. The red color of ripe tomato fruit is due mainly to the accumulation of the carotenoid all-trans-lycopene, which is produced during fruit ripening. In addition to lycopene, tomato fruit contain significant levels of violaxanthin, and lutein. Tomato plants having mutation(s) in the carotenoid pathway have an altered carotenoid composition, which result in different fruit colors, such as orange (tangerine beta) or yellow (r) fruit (Lewinsohn et al. 2005 Trends Food Sci Technol., Vol 16 pp 407-415).
Additionally flavonoids play a role in determining the color of tomato fruit. Flavonoids accumulate predominantly in the fruit peel, since the flavonoid pathway is not active in the fruit flesh. One of the most abundant flavonoids in tomato fruit peel is the yellow-colored naringenin chalcone. In addition, up to 70 different flavonoids have been identified in tomato fruit.
Ballester et al. performed a phenotypic analysis of an introgression line (IL) population derived from a cross between Solanum lycopersicum “Moneyberg” and the wild species Solanum chmielewskii which revealed three ILs with pink fruit color. These ILs had a homozygous S. chmielewskii introgression on the short arm of chromosome 1, consistent with the position of the y (yellow) mutation known to result in colorless epidermis, and hence pink-colored fruit when combined with a red flesh. This same study revealed that the pink fruit lacked the ripening-dependent accumulation of the yellow-colored flavonoid naringenin chalcone in the fruit peel—which increased in the peel of Moneyberg fruit upon ripening-, while carotenoid levels were not affected (Ballester et al. 2010 Plant Physiology, vol 152 pp 71-84). In the same study Ballester et al. disclose (Ballester et al. 2010 Plant Physiology, vol 152 pp 77 right-hand column) that “the deduced amino acid sequence of the pink MYB12 alleles obtained from commercial sources was identical to the red Moneyberg allele”, suggesting “that deregulated MYB12 gene expression”, observed in all pink genotypes tested [by Ballester et al.], rather than aberrant MYB12 [protein] function is the primary cause of the pink phenotype. This cause of the pink color was confirmed using gene-silencing studies genetic mapping, segregation analysis, and VIGS (Virus Induced Gene Silencing) results.
Thus far, analysis of existing commercial non-GMO colorless peel (i.e. pink) y mutant revealed no mutations in the myb12 allele nor in its promotor sequence indicating that the y mutant phenotype is due to a mutation in a regulatory gene i.e. an additional mutant allele (Adato et al 2009 PLoS Genetics vol 5 issue 12 e1000777).
Despite the above, no alternative non-GMO pink mutants for tomato fruit have been found so far.
There is, thus, a need for alternative, non-GMO, cultivated tomato plants producing pink tomato fruit.